| AVS 57th International Symposium & Exhibition | |
| Plasma Science and Technology | Tuesday Sessions |
| Session PS2-TuM |
| Session: | Plasma Diagnostics, Sensors and Control |
| Presenter: | Y. Nakazaki, Tokai University, Japan |
| Authors: | H. Shindo, Tokai University, Japan Y. Nakazaki, Tokai University, Japan |
| Correspondent: | Click to Email |
A new diagnostic tool to measure Electron Energy Distribution Function (EEDF) by an emissive probe has been proposed[1] and applied to radio-frequency (RF) plasmas. In particular, the measurements are made in the capacitive mode which is occurred at the various frequencies of 2 to 60 MHz. It is generally difficult for a conventional probe method to measure EEDF in RF plasmas, because of the plasma potential fluctuation, particularly in the capacitive mode. On the contrary, one of the advantages of the present method is that the measurements are free from the high frequency potential fluctuation.
The method is based on measurement of the functional relationship between the floating potential change ΔVF and the heating voltage VH of emissive probe. If the Maxwellian plasma is concerned, the following equation can be obtained as a practical and useful formula.[1]
It is important to know that the value of ΔVF contains information of electron energy distribution with several electron volt interval along the floating potential VF, because ΔVF is determined only by the current of plasma electrons with an energy interval.
In the experiments, the values of ΔVF were measured in the Ar plasmas which were produced by a single-loop antenna[2] in the frequencies of 2 to 60 MHz and the gas pressures of 5 to 100 mTorr. The values of ΔVF behave quite differently, depending on the frequency and the gas pressure, hence the plasma mode. It is found that the capacitive mode is appeared at the pressures below 20 mTorr at 2 MHz, 10 mTorr at 13 MHz, and at 60 MHz, the behavior of floating potential change ΔVF is fairly complicated, hence non-Maxwellian plasma. In all capacitive modes, from the data set of ΔVF and VF, the electron energy probability function (EEPF) is calculated, and the EEPF thus obtained reveals a bi-Maxwellian with the two electron temperatures depending on the frequencies. For an example, the data set of ΔVF and VF at the pressures of 3 to 7 mTorr at 13 MHz revealed the high energy tail with the temperature of 3.0 to 5.0, while at 10 mTorr the EEPF showed a straight line, hence a Maxwellian. At 2 MHz, on the other hand, the capacitive mode was appeared even in higher RF power, but the two temperature mode was not so typical. It should be emphasized that the present diagnostic method becomes powerful in observation of the plasma mode transition in a variety of frequencies.
References:
[1] K.Kusaba and H.Shindo, Review of Scientific Instruments, 78, 123503-1(2007).